1. Field of the Invention
The present invention relates to an optical fiber having a ring core portion, and a favorable method of making the same.
2. Related Background Art
Dispersion-shifted optical fibers have a zero-dispersion wavelength, where their chromatic dispersion value becomes zero, in the wavelength range of 1.55 xcexcm. Known as one kind thereof are those of a ring-shaped structure in which a ring core region having a higher refractive index and a cladding region having a lower refractive index are disposed concentrically around a center core region. A dispersion-shifted optical fiber having such a ring-shaped structure of refractive index profile is manufactured by drawing an optical fiber preform having a similar refractive index profile.
The refractive index profile of this optical fiber preform can be realized by employing silica glass as a main ingredient, adding F (fluorine) element to a center core portion which is to become the center core region of the optical fiber, and adding GeO2 (germanium dioxide) to a ring core portion which is to become the ring core region. This optical fiber preform is subjected to melt spinning, i.e., so-called drawing, an optical fiber having a desirable refractive index profile may be obtained.
However, upon drawing, the optical fiber preform is heated and, at this time, the F element added to the center core portion diffuses into its surrounding regions, whereas Ge added to the ring core portion diffuses into the center core portion and the outer cladding region. Due to this mutual diffusion, transmission loss would increase. Also, when Ge and F mingle with each other, then GeF4 or GeO may be produced in a heating and unifying process, so as to generate bubbles, which deteriorate the quality of the optical fiber to be made. As a result, there has been a problem that desirable fiber characteristics may not be obtained.
In order to overcome the above-mentioned problem, it is an object of the present invention to provide an optical fiber of a ring-shaped structure having a region doped with F element and a region doped with Ge, which can be manufactured stably and has a desirable refractive index profile; and a method of making the same.
For overcoming the above-mentioned problem, the optical fiber in accordance with the present invention is composed of silica glass, having a center core region doped with F element and a ring core region doped with GeO2, wherein a buffer layer made of undoped SiO2 (silicon dioxide) or SiO2 doped with both or one of P (phosphorus) and Cl (chlorine) is disposed between the center core region and the ring core region.
The optical fiber in accordance with the present invention can be made by drawing an optical fiber preform having a cross-sectional profile similar to that of this optical fiber. Namely, the optical fiber preform also has a buffer layer made of undoped SiO2 or SiO2 doped with one or both of P and Cl. When this optical fiber is drawn, the mutual diffusion of F and Ge between the ring core region and the center core region is suppressed due to the existence of buffer layer. The fact that this buffer layer exists in the resulting optical fiber indicates that the mutual dispersion suppressing effect sufficiently functions, so as to maintain fiber characteristics.
Preferably, the thickness of the buffer layer is at least 0.01 xcexcm but not greater than 5 xcexcm. If the buffer layer is thinner than the lower limit of this range, there is a possibility of the mutual diffusion of Ge and F occurring beyond the buffer layer. On the other hand, if the buffer layer is thicker than the upper limit of the range, the bending loss occurring when the optical fiber is bent may become unfavorably large.
Alternatively, the optical fiber in accordance with the present invention is composed of silica glass, having a center core region, a ring core region doped with GeO2 (germanium dioxide), and an inner cladding region doped with F (fluorine) element which are arranged concentrically, wherein a buffer layer made of undoped SiO2 or SiO2 doped with both or one of P and Cl is disposed between the ring core region and the inner cladding region.
The mutual diffusion of F and Ge between the ring core region and the inner cladding region when drawing an optical fiber preform having a similar structure is suppressed due to the existence of buffer layer in this case as well. Similarly, the fact that this buffer layer exists in the resulting optical fiber indicates that the mutual dispersion suppressing effect sufficiently functions, so as to maintain fiber characteristics.
In this case, the thickness of the buffer layer is preferably 0.01 xcexcm or greater. It is because of the fact that there is a possibility of the mutual diffusion of Ge and F occurring beyond the buffer layer if the buffer layer is thinner than this lower limit. On the other hand, the change in bending loss occurring upon bending the optical fiber depending on the thickness of the buffer layer in this case is smaller than that in the case where the buffer layer is formed between the ring core region and the center core region, whereby the buffer layer can be made thicker.
Alternatively, the optical fiber in accordance with the present invention is comprised of silica glass, having a center core region doped with F element and a ring core region doped with GeO2; wherein, letting a [xcexcm] be the radius of the center core region, and CG(r) [wt %] be the concentration of GeO2 in the ring core region at a position separated from the center by a radius r [xcexcm], the concentration gradient YG1 [wt %xc2x7xcexcm2] of GeO2 in a boundary portion of the ring core region with respect to the center core region defined by:                               y          G1                =                              ∫            a                          a              +              1                                ⁢                                    (                                                                    rC                    G                                    ⁡                                      (                    r                    )                                                  ⁢                                  exp                  ⁡                                      (                                          a                      -                      r                                        )                                                              )                        ⁢                          ⅆ              r                                                          (        1        )            
is set to 100 wt %xc2x7xcexcm2 or less; or, letting CF(r) [wt %] be the concentration of F element in the center core region at a position separated from the center by a radius r [xcexcm], the concentration gradient yF1 [wt %xc2x7xcexcm2] of F element in a boundary portion of the center core region with respect to the ring core region defined by:                               y          F1                =                              ∫                          a              -              1                        a                    ⁢                                    (                                                                    rC                    F                                    ⁡                                      (                    r                    )                                                  ⁢                                  exp                  ⁡                                      (                                          r                      -                      a                                        )                                                              )                        ⁢                          ⅆ              r                                                          (        2        )            
is set to 18 wt %xc2x7xcexcm2 or less.
Alternatively, the optical fiber in accordance with the present invention is composed of silica glass, having a center core region, a ring core region doped with GeO2, and an inner cladding region doped with F element which are arranged concentrically; wherein, letting b [xcexcm] be the radius of the ring core region, and CG(r) [wt %] be the concentration of GeO2 in the ring core region at a position separated from the center by a radius r [xcexcm], the concentration gradient YG2 [wt %xc2x7xcexcm2] of GeO2 in a boundary portion of the ring core region with respect to the inner cladding region defined by:                               y          G2                =                              ∫                          b              -              1                        b                    ⁢                                    (                                                                    rC                    G                                    ⁡                                      (                    r                    )                                                  ⁢                                  exp                  ⁡                                      (                                          r                      -                      b                                        )                                                              )                        ⁢                          ⅆ              r                                                          (        3        )            
is set to 180 wt %xc2x7xcexcm2 or less; or, letting CF(r) [wt %] be the concentration of F in the inner cladding region at a position separated from the center by a radius r [xcexcm], the concentration gradient YF2 [wt %xc2x7xcexcm2] of F element in a boundary portion of the inner cladding region with respect to the ring core region defined by:                               y          F2                =                              ∫            b                          b              +              1                                ⁢                                    (                                                                    rC                    F                                    ⁡                                      (                    r                    )                                                  ⁢                                  exp                  ⁡                                      (                                          b                      -                      r                                        )                                                              )                        ⁢                          ⅆ              r                                                          (        4        )            
is set to 30 wt %xc2x7xcexcm2 or less.
The diffusion velocity of the above-mentioned mutual diffusion of Fe and Ge generated upon drawing results from the concentration gradient in the boundary portion between the region doped with F element and the region doped with GeO2. The inventors have found that, when the radial distribution of doping amount of GeO2 or F element in each region is set such that the weighted concentration gradients yG1, yF1, yG2, yF2 in boundary portions defined by equations (1) to (4) become predetermined values or less, the diffusion velocity of F and Ge lowers, thereby suppressing the mutual diffusion. As a consequence, desirable fiber characteristics are reliably obtained.
On the other hand, the method of making an optical fiber in accordance with the present invention comprises steps of: (1) preparing a silica glass pipe having a layer doped with GeO2 at least on an inner periphery side thereof; (2) depositing undoped SiO2 or SiO2 doped with one or both of P and Cl onto the inside of the silica glass pipe, so as to produce a buffer layer; (3) inserting a silica glass rod doped with F element into the inside of the buffer layer, and then heating and unifying the silica glass rod and the buffer layer, so as to prepare an intermediate preform; and (4) melt-spinning an optical fiber preform including the intermediate preform. According to this method, an optical fiber having the above-mentioned buffer layer can be made favorably.
Alternatively, the method of making an optical fiber in accordance with the present invention comprises steps of: (1) preparing a silica glass pipe having a layer doped with GeO2 at least on an inner periphery side thereof; (2) heating the silica glass pipe, so as to evaporate GeO2 in its inner surface and eliminate at least a part thereof; (3) inserting a silica glass rod doped with F element into the inside of the silica glass pipe, and then heating and unifying the silica glass rod and the silica glass pipe, so as to prepare an intermediate preform; and (4) melt-spinning an optical fiber preform including the intermediate preform. According to this method, the above-mentioned optical fiber in which the germanium dioxide concentration in the vicinity of the boundary portion of the ring core region is lowered can be made favorably.
The method of making an optical fiber having an inner cladding region in accordance with the present invention includes the following three methods. The first method comprises steps of: (1) preparing a silica glass pipe having a layer doped with F at least on an inner periphery side thereof; (2) depositing undoped SiO2 or SiO2 doped with one or both of P and Cl onto the inside of the silica glass pipe, so as to form a buffer layer; (3) further forming at an inner peripheral portion thereof a glass layer doped with GeO2, so as to produce an intermediate pipe; (4) inserting a silica glass rod into the inside of the intermediate pipe, and then heating and unifying the silica glass rod and the intermediate pipe, so as to prepare an intermediate preform; and (5) melt-spinning an optical fiber preform including the intermediate preform.
The second method comprises steps of: (1) concentrically forming, successively from the axial center side, a silica layer, a layer doped with GeO2, and an undoped layer or a layer doped with one or both of P and Cl, so as to prepare a silica glass rod; (2) preparing a silica glass pipe having a layer doped with F element at least on an inner periphery side thereof; (3) inserting the silica glass rod into the silica glass pipe, and then heating and unifying the silica glass rod and the silica glass pipe, so as to prepare an intermediate preform; and (4) melt-spinning an optical fiber preform including the intermediate preform.
The third method comprises the steps of (1) and (2) steps of the first method; (3) forming a silica glass rod having a silica layer at the axial center and a layer doped with germanium dioxide therearound; (4) inserting the silica glass rod into the inside of the buffer layer, and then heating and unifying the silica glass rod and the buffer layer, so as to prepare an intermediate preform; and (5) melt-spinning an optical fiber preform including the intermediate preform. According to any of these methods, the optical fiber in accordance with the present invention having a buffer layer between the inner cladding region and the ring core region can be made favorably.
In the step of preparing the silica glass pipe in these methods, a silica glass rod having a layer doped with fluorine element or germanium dioxide at least on the inner periphery side thereof may be plastically deformed at a high temperature, so as to form an opening penetrating through the axial center, thereby yielding a glass pipe. Hence, a required silica glass pipe can be formed favorably.
Another method of making an optical fiber in accordance with the present invention comprises steps of: (1) preparing a silica glass rod having a silica layer on the axial center side and a ring-shaped layer doped with germanium dioxide disposed on the outside thereof; (2) opening the silica glass rod along the axis, so as to prepare a silica glass pipe; (3) inserting a silica glass rod doped with fluorine element into the inside of the silica glass pipe, and then heating and unifying the silica glass rod and the silica glass pipe, so as to form an intermediate preform; and (4) melt-spinning an optical fiber preform including the intermediate preform. The above-mentioned optical fiber having a buffer layer can favorably be made by this method as well.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.